The present invention relates to droplet deposition apparatus, in particular an inkjet printhead, which comprise a channel communicating with a supply of droplet liquid and an opening for ejection of droplets therefrom, at least one channel side wall being displaceable in response to electrical signals, thereby to effect ejection of droplets from the channel.
FIG. 1a is a cross-sectional view of the channels of the prior art inkjet printhead construction according to WO92/22429. Piezoelectric ceramic sheet 12 is poled in its thickness direction 17 and formed in one surface with channels 11 bounded on two sides lying parallel to the channel axis by channel walls 13. By means of electrodes 23 formed on either side of each wall 13, an electric field can be applied to the piezoelectric material of the walls, causing them to deflect in shear mode in a direction transverse to the channel axis. Pressure waves are thereby generated in the ink which result in the ejection of an ink droplet. These principles are known in the art, e.g. from EP-A-0 364 136.
Channels 11 are closed along one side lying parallel to the channel axis by the surface of a cover 14 having conductive tracks 16 at the same pitch interval as the ink channels formed thereon. Solder bonds 28 are formed between tracks 16 and the channel wall electrodes 23, thereby securing the cover to the base and creating an electrical connection between the electrodes and the track in a single step. To protect them from later being corroded by the ink, electrodes and tracks are then given a passivant coating.
As shown in FIG. 1b, which is a sectional view taken along the longitudinal axis A of a single channel of the prior art printhead of FIG. 1a, a nozzle plate 20 having respective ink ejection nozzles 22 is mounted at the front of the sheet 12 whilst an ink manifold 26 is defined at the rear by a manifold structure 21. Tracks 16 are led to the rear of cover 14 for connection to a drive circuit, typically embodied in a microchip 27 which in turn is driven by signal received via input tracks 18.
In printheads of this ilk, the channel walls and in particular the electrodes formed thereonxe2x80x94are often passivated so as to protect from subsequent corrosion by the ink. Reference is made in this regard to WO95/07820.
In the device discussed above, however, such conventional passivation prior to attachment of the cover would inhibit the formation of solder bonds between the electrodes and the tracks. On the other hand, passivation after the cover has been attached can only be applied from the end of the channel, resulting in low quality coating of the electrodes and tracks, especially at the midpoint of the channel remote from the channel ends.
The present invention has as an objective a printhead construction that retains the connection advantages associated with the conductive tracks formed on the cover of the prior art construction and yet is amenable to passivation.
Accordingly, the present invention consists in one aspect in droplet deposition apparatus comprising at least one channel having means for communicating with a supply of droplet liquid and an opening for ejection of droplets;
the channel being bounded on at least one side lying parallel to the channel axis by a channel wall associated with actuator means; the actuator means effecting displacement of the channel wall in response to electrical signals, thereby to effect ejection of droplets from the channel;
the channel being bounded on a further side lying parallel to the channel axis by a cover surface, the cover surface having formed thereon at least one conductive track for conveying electrical signals to said actuator means, the point of electrical connection between the track and the actuator means lying outside the channel.
Since the sole point of electrical connection between the track and the actuator in accordance with the present invention lies outside of the channel and thus out of contact with the ink (with its potentially corrosive effects), passivation of this point is no longer required. The channel itself can therefore be conventionally passivated via the open tops of the channels, thereafter, the cover can be attached and electrical contact established between the conductive tracks on the cover and the actuator means associated with the channel walls. Even in a printhead thatxe2x80x94because of the type of ink it is designed to firexe2x80x94does not require passivation, a point of electrical connection lying outside the channel as per the present invention is less likely to fail in fatigue than the channel-length solder bonds of the prior art device of FIGS. 1a, 1b. 
A corresponding method according to a first aspect of the invention consists in a method of manufacture of droplet deposition apparatus method of manufacture of droplet deposition apparatus, the method comprising the steps of:
forming in a base component at least one open-topped channel and, bounding said channel on at least one side lying parallel to the channel axis, a channel wall associated with actuator means for effecting displacement of the channel wall in response to electrical signals, thereby to effect ejection of droplets from the channel;
closing the channel on a further side lying parallel to the channel axis by a cover surface, the cover surface having formed thereon at least one conductive track for conveying electrical signals to said actuator means; and
electrically connecting the conductive track and the actuator means at a point lying outside the channel.
Advantageously, the step of closing the channel results in the electrical connection of the conductive track and the actuator means, thereby simplifying the manufacturing process.
The first aspect of the invention also consists in droplet deposition apparatus comprising: a bottom sheet of piezo-material poled droplet deposition apparatus comprising:
a bottom sheet of piezo-material poled in a direction normal to said sheet and formed with a multiplicity of parallel, open-topped channels mutually spaced in an array direction normal to the length of the channels and defined each by facing side walls and a bottom surface extending between said side walls;
a top sheet facing said bottom surfaces of said channels and bonded to said side walls to close said channels at the tops thereof;
respective nozzles communicating with said channels for the ejection of droplets of liquid therefrom;
connection means for connecting said channels with a source of droplet deposition liquid;
wherein each channel is formed with a forward part in which electrodes are provided on opposite sides of at least one of the side walls defining the channel, thereby to form a shear mode actuator for effecting droplet expulsion from the channel; and
wherein each channel is formed with a rearward part having an electrically-conductive coating which is in electrical contact with the at least one electrode on the channel-facing sides of the side walls in the forward part;
sealing means separating the forward part from the rearward part; and wherein
the apparatus further comprises conductive tracks formed on that surface of said top sheet that is bonded to said side walls, the conductive tracks being in electrical contact with the electrically-conductive coating in said rearward part.
A corresponding method comprises the steps of forming a bottom sheet with a layer of piezo-material poled in a direction method of manufacture of a droplet deposition apparatus comprising the steps of:
forming a bottom sheet with a layer of piezo-material poled in a direction normal to said sheet;
forming a multiplicity of parallel, open-topped channels mutually spaced in an array direction normal to the length of the channels, each channel being defined by facing side walls and a bottom surface extending between said side walls, each channel further having a forward part and a rearward part;
forming electrodes on opposite sides of at least one of the side walls defining the forward part of each channel, thereby to form a shear mode actuator for effecting droplet expulsion from the channel; and
forming in the rearward part of each channel an electrically-conductive coating in electrical contact with a respective electrode;
providing a top sheet having a surface formed with conductive tracks thereon; and
bonding that surface of the top sheet having conductive tracks thereon to said side walls so as to close said channels at the tops thereof;
establishing electrical contact between said tracks and the respective electrically-conductive conductive coating of each channel; and
providing sealing means separating the forward and rearward parts of each channel.
A second aspect of the present invention consists in droplet deposition apparatus comprising droplet deposition apparatus comprising:
at least one longitudinal, open-topped droplet liquid channel defined by facing longitudinal side walls and a bottom, longitudinal surface extending between the side walls;
means for applying an electric field to piezoelectric material in at least one of said walls, thereby to effect displacement of the wall relative to said longitudinal channel so as to eject a droplet from the channel; and
a cover closing the open, longitudinal top side of the channel;
wherein said bottom longitudinal surface of the channel is formed with an opening for droplet ejection, and;
the cover incorporates two ports for supply of droplet liquid, the ports being spaced along the channel on either side of the opening.
Such a construction again simplifies the manufacture of known printheads, particularly those of the xe2x80x9ctop shooterxe2x80x9d kind discussed in WO91/17051. FIG. 2 shows a sectional view along the channels of such a prior art printhead, with those features that correspond to FIG. 1 being denoted by corresponding reference numbers. Droplet ejection takes place from a nozzle 22 formed in the channel cover component 60 whilst droplet liquid is supplied to the channel via ports 33 formed in the channel base and which are typically connected in their turn to ink supply conduits (not shown) formed in a base component 35 that is separate from the piezoelectric channeled component 12.
In accordance with the invention, an opening communicating with a droplet ejection orifice is formed in the bottom surface of the channel, thereby allowing the cover component to incorporate ports for supply of ink into the channel. A further, separate base component is consequently no longer required.
A third aspect of the invention comprises droplet deposition apparatus comprising:
at least one longitudinal, open-topped droplet liquid channel defined by facing longitudinal side walls and a bottom, longitudinal surface extending between the side walls;
means for supplying droplet liquid to the channel;
means for applying an electric field to piezoelectric material in at least one of said walls, thereby to effect displacement of the wall relative to said longitudinal channel so as to eject a droplet from the channel; and
a cover closing the open, longitudinal top side of the channel;
wherein the bottom longitudinal surface of the channel is formed with two openings for droplet ejection, the openings being spaced along the channel.
Such a construction brings to the arrangement of PCT application no. PCT/GB98/01495 the aforementioned advantage of reduced component count.
Corresponding method claims are also comprised in the present invention, and other aspects are as set out in other independent claims.
Further advantageous embodiments of the invention are set out in the description, drawings and dependent claims.
The disclosure of all claims is deemed incorporated here as consistory clauses, unless already set out above.
The invention will now be described by way of example by reference to the following diagrams, of which: